OK, you forced me to look it up. We know Saturn isn’t antimatter because if it were the solar wind’s reaction with Saturn would make it a strong gamma ray source[sup]1[/sup]. It isn’t, so it isn’t.
We are also pretty sure that our galaxy is entirely matter because the cosmic rays very rarely produce antihelium nuclei (alpha particles) when colliding with the interstellar gas. So the detection of even one alpha particle in the cosmic rays would indicate a source of antimatter somewhere in the galaxy. None has been detected so far. Anti protons are produced by the primary cosmic ray collisions with the gas and such particles are occasionally detected in secondary cosmic radiation.[sup]1[/sup]
Based on similar reasoning about other galaxies and clusters of galaxies it is thought that there is practically no permanent antimatter in the universe. There are occasional antimatter particles produced but these annihilate very quickly to produce gamma rays.
Antimatter article by Gary Stiegman of Ohio State University in The Encyclopedia of Physics.